1. Field of the Invention
The present invention relates to an optical cable laid in the underground, on the ground, in the air, or on the sea bottom; and a chamber element utilized when optical fibers are mounted in the optical cable.
2. Related Background Art
Conventionally known as a technique in such a field is that disclosed in Japanese Patent Application Laid-Open No. 55-45087. The optical cable disclosed in this publication comprises a plurality of chamber elements made of a plastic each provided with a fiber containing cavity for containing a coated optical fiber, a loose tube, a ribbon fiber, or the like (hereinafter simply referred to as "optical fiber" in general terms); and a tension member functioning as a central member. The individual chamber elements containing optical fibers are assembled around the central member in the state where their bottom faces are in contact with the central member. Each chamber element has a bottom part and a pair of side wall parts rising from both ends of the bottom part. Namely, the chamber element has substantially a U-shaped cross section. The chamber element is extrusion-molded as a straight elongated member. It is considered preferable that the bottom part and side wall parts of the chamber element each have a thickness of 0.5 mm or greater.
Also, Japanese Patent Application Laid-Open No. 4-182611 discloses an optical cable in which a plurality of chamber elements each having substantially a U-shaped cross section and containing an optical fiber therein are bent along the outer periphery of a central member and are assembled in an S-Z strand around the central member. In this case, as shown in FIG. 18, in order for fiber containing cavities 102 to face outward, chamber elements 100 are assembled around the central member 110 in the state where their bottom faces are in contact with the central member 110. Hence, in the S-Z locus defined by each chamber element 100, at each position Re (hereinafter referred to as "S-Z reverse portion"; see FIG. 19) where S strand turns into Z strand or vice versa, it is necessary to bend the chamber element 100 in the widthwise direction x of its fiber containing cavity 102 as shown in FIG. 20. On the other hand, at each intermediate position (hereinafter referred to as "S-z transit portion") between adjacent S-Z reverse portions, it is necessary to bend the chamber element 100 in the depth direction y of its fiber containing cavity 102 as shown in FIG. 21.
The conventional optical cables and chamber elements, however, have the following problems. Namely, in the case where wide chamber elements each having a fiber containing cavity whose width is greater than its depth are employed and assembled around a central member in an S-Z strand, the chamber element to be bent in the width direction x at S-Z reverse portions Re (see FIG. 20) may bend in the depth direction y (see FIG. 21 or 22). In this case, the chamber element may be twisted such that a side face thereof comes into contact with the central member as shown in FIGS. 23 and 24, whereby the fiber containing cavity fails to face correctly outward (this state being hereinafter referred to as "turn-over").
On the other hand, in the case where elongated chamber elements each having a fiber containing cavity whose depth is greater than its width are employed and assembled around a central member in an S-Z strand, the chamber element to be bent in the depth direction y at S-Z transit portions Tr (see FIG. 21 or 22) may bend in the widthwise direction x (see FIG. 20). Also in this case, the chamber element may be twisted such that a side face thereof comes into contact with the central member as shown in FIGS. 23 and 24, whereby the fiber containing cavity fails to face correctly outward.
In either case, when the chamber element is turned over, a side face thereof comes into contact with the central member, whereby the fiber containing cavity fails to face correctly outward. Then, the optical fiber contained in the fiber containing cavity of the chamber element is twisted together with the chamber element. As a result, so-called microbend may occur in the optical fiber, thereby enhancing its transmission loss. Also, in the case where, after an optical cable is laid, optical fibers are to be taken out from the optical cable so as to branch out, if a chamber element is turned over, it will be difficult to take out the optical fibers, thus lowering workability.